The present invention relates to an image forming material that can be formed into an image by electro-photography, thermal transfer recording, writing tools or printing, and that can be decolored by control of thermal hysteresis or by contact with a solvent.
With the recent progress of office automation, the amounts of various pieces of information are significantly increasing, and information output is increasing accordingly. Information output is represented by display output and hard copy output from printers to paper sheets. However, display output requires a large-scale circuit board in a display unit and hence has problems in portability and cost. Hard copy output uses a large amount of paper as a recording medium when the information amount increases, leading to problems about the protection of resources. In addition, in order to recycle paper sheets on which images are printed with a printer or a copier, it is necessary to use a large amount of bleaching agent and water, which brings about a rise in recycling cost. Thus, it is proposed to reduce consumption of paper sheets substantially by printing an image on a paper sheet using a decolorable image forming material, decoloring the formed image to return the paper sheet to a blank sheet, reusing the blank sheet, and recycling the sheet at the time when damage of the sheet becomes serious so that the sheet cannot be reused.
Recently, rewritable paper has been proposed in order to reuse hard copy paper sheets. However, this rewritable paper is applicable only to thermal recording and is not recyclable because it is made of special paper, although it is reusable.
Heretofore, an image forming material that can be decolored by heating has been proposed in, for example, Published Unexamined Japanese Patent Application No. 7-81236. The image forming material includes a color former such as a leuco dye, a developer, and an organophosphoric compound having decoloring ability. When such an image forming material is used, however, decoloration is insufficient and, as a result, a paper sheet is hard to return to the blank state.
We have been developing a decolorable image forming material in which a color former, a developer and a decolorant compatible with the developer are dispersed in a binder resin. Now, it becomes possible to obtain an excellent decolored state in various material systems by heating or by contact with a solvent. However, since some of the decolorants showing excellent performance are relatively expensive, it is desirable to reduce the amount of decolorant used.
As for decoloration of an image of a decolorable image forming material, it is found that the amount of the decolorant required to obtain an excellent decolored state can be made smaller in solvent decoloration than in thermal decoloration. Therefore, the amount of the decolorant contained in the decolorable image forming material is determined based on the amount required for the thermal decoloration. However, when the amount of the decolorant in the image forming material is excessively large, reflection density of a developed image may vary depending on a manufacturing lot because dispersion of the components in a matrix material is made inhomogeneous, or fixing defects may be caused due to lack of the matrix material.
If paper sheets are reused in an office where paper consumption is large, use of a solvent is undesirable from the viewpoint of, for example, its smell. Further, it is preferable to heat paper sheets stacked into a bundle so as to handle a large amount of paper sheets at one time. However, such processing conditions or decolorable image forming materials appropriate for those conditions have not been studied so far, and, therefore, it is found that an excellent decolored state cannot be obtained in some cases.